The present invention relates to a tensioning arrangement for a power-transmitting element incorporated in a transmission, and is intended to be driven in an endless path.
In internal combustion engines chain transmissions are often used for driving the camshaft of the engine via its crankshaft. In order to compensate for chain wear, it is customary to use a tensioning arrangement which automatically presses against the chain and tensions it.
The force which is required to tension the chain is generated in commercially available tensioning arrangements by means of a piston which is displaceably arranged in a cylinder and to which is conveyed by a spring element, for example a compression spring, a movement projecting it out of the cylinder to tension the chain.
In this connection the piston cooperates with a locking arrangement which allows the piston to project out of the cylinder in order to tension the chain, but prevents the piston from moving back into the cylinder and thus causing the chain to slacken.
In known solutions of the type in question the chain acquires a certain over-tensioning. This is regarded as being necessary in order to ensure that the chain will be sufficiently tensioned under all operating conditions.
Known solutions have a number of disadvantages, of which it may be mentioned that the over-tensioning contributes to the noise intensity increasing in the chain transmission and to the wearing of the chain being accelerated.
The disadvantages are pronounced in the case of chain transmissions for driving the camshaft of the engine via its crankshaft. Since both the noise intensity and the wearing of the chain increase with increased chain speed, the disadvantages are even more pronounced in the case of chain transmissions for driving vibration-damping balance shafts arranged on the engine via its crankshaft. A chain in such transmissions is in fact driven at a speed which is considerably higher than the speed at which the chain in camshaft transmissions is driven.
The foregoing disadvantages have been observed by a number of users of tensioning arrangements, but, since no solution to the problems has been forthcoming, certain users have abandoned automatically adjustable tensioning arrangements in favour of manually adjustable tensioning arrangements which must be adjusted "by hand" at regular intervals.
The object of the present invention is to eliminate the abovementioned disadvantages. For this purpose the invention provides a tensioning arrangement for a power-transmitting element incorporated in a transmission and intended to be driven in an endless path. The tensioning arrangement comprises a piston which is displaceable in a cylinder and which, in the cylinder, delimits a space and which directly or indirectly bears against the power-transmitting element; and a spring element which imparts to the piston a first force for conveying to the piston a movement directed towards the power-transmitting element. The invention is characterized in that the space delimited by the piston is connected to a pressure source which only supplies pressure medium to the space when the transmission is in operation, by which means a second force which is directed counter to the first force acts on the piston and conveys to it a limited movement for reduced force effect on the power-transmitting element.
The tensioning arrangement according to the invention provides for an over-stretching of the chain when the engine is not in operation. On the other hand, when the engine is started up, the chain tensioning is reduced to a tensioned but not over-tensioned state. By this means the noise intensity in the chain transmission decreases at the same time as the wearing of the chain is reduced.
In advantageous embodiments the inward movement of the piston is limited by a locking arrangement which cooperates with the piston and which is movable between a first and a second end position in a bore in the tensioning arrangement, which bore connects the oil system of the vehicle engine to the space delimited by the piston. In this way the oil pressure of the engine is used in order, via a locking arrangement with predetermined play, to reduce the chain tensioning to a tensioned but not over-tensioned state.